The Influence of Surface Roughness on Flow Transition by Well-Resolved Large Eddy Simulations

Abstract

Abstract The transition of a spatially developing boundary layer is studied through highly resolved Large Eddy Simulations (LES) when subjected to external disturbances in the presence of surface roughness. Three rough wall conditions are considered in the present analysis, where roughness elements are homogeneous in the spanwise direction with cross-sectional shapes taken as triangle, square and semi-circle. The aspect ratio (s/k) considered here is 3, where k is the roughness height, and s is the spacing between the consecutive rows of roughness elements in the streamwise direction. The inlet flow Reynolds number based on the momentum thickness and freestream velocity (Reθ) is 360, while the freestream turbulence (fst) is 2.5%. The transition mechanisms are observed to change significantly with roughness shapes. Attributing to the sharp tip of the triangular shape, the shear layer trips to transition from laminar to turbulent flow via Kelvin-Helmholtz (K-H) instability. Whereas Λ-vortices and longitudinal streaks are the geneses for the boundary layer transition in square and semi-circular elements, respectively. Further, the influence of roughness on transition onset is explored by analyzing the turbulence statistics such as rms quantities, shear stress and spectral response.